I. Field of the Invention
The present invention relates generating and distributing a secret key for symmetric encryption by employing channel characteristics of the communication channel between a pair of transceivers.
II. Description of the Related Art
While a wireless communication link is vulnerable to interception by unintended receivers, the physical characteristics of the propagation channel (such as reciprocity, highly variable channel distortions, and the uniqueness of those distortions in any given link) can be exploited for generating and exchanging encryption keys, and encrypting transmissions.
Techniques for key generation commonly employ random processes. The physical wireless channel provides the required statistical randomness, and channel reciprocity (i.e., radio waves propagating in both directions of a radio link between a pair of transceivers experience the same multipath channel distortions if their frequencies are the same) potentially enables secure key exchange without transmitting key information that can be intercepted by an unintended receiver. Specifically, each of a pair of transceivers observes a random process (i.e., their propagation channel) that is observed differently by any unintended receiver. For example, each transceiver estimates the common channel from known training signals received from the other transceiver. Temporal and spatial variations of the channel are random. Thus, for transceivers that are sufficiently distant from each other (e.g., typically a few wavelengths), their channel transfer functions are uncorrelated.
Practical examples of where reciprocity can be achieved include wireless systems employing time-division duplex (TDD), such as 802.11, 802.16 (WiMAX), and LTE. However, some aspects of the invention provide for full-duplex operation in non-TDD systems by cancelling transmit signal leakage into the receiver, which enables a pair of transceivers to simultaneously transmit and receive signals in the same frequency band.